The peripheral auditory system David Meredith Aalborg University.

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Presentation on theme: "The peripheral auditory system David Meredith Aalborg University."— Presentation transcript:

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The peripheral auditory system David Meredith Aalborg University

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Linearity

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Linearity in acoustic systems

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Filters

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Real filters

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Impulse response Pass a signal with a flat spectrum through a filter, then output will have same shape as filter characteristic Click or impulse has a flat spectrum Narrow band-pass filter has “ringing” response because output resembles a long-lasting sinuoid (which has a pure line spectrum) Broader the bandwidth, the more the output resembles a click Narrow bandwidth = good frequency resolution but poor time resolution Broad bandwidth = good time resolution but poor frequency resolution Ear has both good time resolution and good frequency resolution...

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The function of the middle ear Middle ear acts as an impedance matching device that allows more sound to be transmitted to the cochlea If sound impinged directly on oval window, them most would be reflected because of high acoustic impedance of cochlear fluids and oval window Oval window is 1/25 area of tympanum – suggests that pressure on oval window is 25 times that on tympanum (i.e., 625 times the intensity) But ossicles amplify sound by 2: so amplitude multiplied by 50 times – implies 2500 times intensity or 30dB increase!

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Cochlea End near oval window called basal end, other end called apical end Basal end responds to high frequencies, apical end responds to low frequencies

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Basilar membrane Flick a string, then a wave travels along it from the flicked end Same on basilar membrane Stiffness of BM decreases as move away from basal end (oval window) Displacement increases until get to point on BM tuned to frequency of input, then decreases rapidly Figure shows BM at two instants in time when disturbed by a 200Hz sine wave Wave disturbance describes an amplitude envelope

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Frequency resolution on BM Each point on the BM behaves like a bandpass filter 3dB down bandwidth hard to measure on BM, so measure 10dB down bandwidth instead Ratio of bandwidth to centre frequency (relative bandwidth) approximately constant along BM Reciprocal of rel. bandwidth is called Q which is measure of sharpness of tuning

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Frequency resolution on BM Actual resolution better because – BM is non-linear: response to quiet sounds not predictable by response to very loud ones – Response is physiologically vulnerable: tuning gets less sharp after death - tuning in living animals much sharper than in dead ones – Response influenced by active processes efferent signals from the cortex